Techniques for producing an electrode for fuel cell are known in the art as follows:
For example, a method for producing an electrode has been proposed by P. Driven et al., wherein water repellency treated carbon powders are applied on a water repellency treated porous carbon paper to form an intermediate layer, on which layer a mixture of catalyst and polymer electrolyte, i.e., high EW ionomer (e.g., EW=1,100), is then applied thinly, thereby preparing an electrode.
Then, a commercial electrolyte membrane is placed between the anode and the cathode as prepared, and hot-pressing is performed at above glass transition temperature of the electrolyte and under a certain pressure to produce a MEA <see P. Driven and W. Engelen, U.S. Pat. No. 5,561,000 (1996)>.
In producing an electrode using such method, an addition of a nafion ionomer to the electrode catalyst layer makes three-interface advantageously increased. Further, the prevention of the flooding comes to be possible.
However, such method has problems that due to the poor water absorbency of the nafion ionomer, water produced by electrochemical reaction in cathode can not be used sufficiently so that the non-humidification operation is substantially impossible to thus require an external humidifier, which correspondingly causes additional costs and volume increase.
Furthermore, such method has another problems that due to the insufficient indirect annealing of electrode by hot-pressing, the nation ionomer is leaked together with the catalyst during the operation of the fuel cell, which is disadvantageous in long-term operation of the fuel cell.
Meanwhile, a three-layered structure catalyst layer of a cathode electrode has been proposed by Yoshitake et al., wherein the first catalyst layer directly contacting the first electrolyte membrane includes an ion conductive polymer ionomer with high ion exchange capacity, i.e., with low EW, the second catalyst layer adjacent to the first catalyst layer includes an ion conductive polymer ionomer with high oxygen solubility, and the third catalyst layer includes soluble fluoropolymer <see M. Yoshitake, I. Terada, H. Shimoda, A. Watanabe, K. Yamada, K. Min and Y. Kunisa, “2002 Fuel Cell Seminar Abstracts”, 826(2002)>.
In producing an electrode using such method, a nafion ionomer with high EW is added to the first and second catalyst layers so that in comparison with the prior electrode, interface resistance between the electrode and the electrolyte membrane can be advantageously reduced and the oxygen concentration in the catalyst layer can be increased to improve the performance of the fuel cell.
However, such method has problems that due to the generation of flooding resulted from high water absorbency, the performance can be abruptly reduced. Further, the method still has problems that due to the insufficient indirect annealing of electrode by hot-pressing, the nation ionomer comes to be leaked together with the catalyst during the operation of the fuel cell, which is disadvantageous in long-term operation of the fuel cell.
Specifically, according to the above prior methods, the MEA was produced with the hot pressing under the conditions of 120˜140° C. temperature, 100˜200 atm pressure and 30 seconds to 2 minutes time. Since the phase inversion of the nation ionomer in the electrode catalyst layer does not occur sufficiently under such conditions, the nafion ionomer serving to transfer hydrogen ion between the electrode and the electrolyte membrane and to bind the catalyst comes to be re-solved by wet reaction gas and leaked out together with the catalyst during the operation of the fuel cell, so that the electrode and the electrolyte membrane are separated from each other to increase resistance, thereby causing the decrease of the performance of fuel cell and giving disadvantageous effects to the long-term operation thereof.